Protein Kinases are receptor type or non-receptor type proteins, which transfer the terminal phosphate of ATP to aminoacid residues, such as tyrosine, threonine, serine residues, of proteins, thereby activating or inactivating signal transduction pathways. These proteins are known to be involved in many cellular mechanisms, which in case of disruption, lead to disorders such as abnormal cell proliferation and migration as well as inflammation.
As of today, there are over 500 known Protein kinases. Included are the well-known Abl, Akt1, Akt2, Akt3, ALK, Alk5, A-Raf, Axl, B-Raf, Brk, Btk, Cdk2, Cdk4, Cdk5, Cdk6, CHK1, c-Raf-1, Csk, EGFR, EphA1, EphA2, EphB2, EphB4, Erk2, Fak, Fes, Fer, FGFR1, FGFR2, FGFR3, FGFR4, Flt-3, Fms, Frk, Fyn, Gsk3a, Gsk3B, HCK, Her2/Erbb2, Her4/Erbb4, IGF1R, IKK beta, Irak4, Itk, Jak1, Jak2, Jak3, Jnk1, Jnk2, Jnk3, KDR, Kit, Lek, Lyn, MAP2K1, MAP2K2, MAP4K4, MAPKAPK2, Met, Mer, MNK1, MLK1, mTOR, p38, PDGFRα, PDGFRβ, PDPK1, PI3Kα, PI3Kβ, PI3Kδ, PI3Kγ, Pim1, Pim2, Pim3, PKC alpha, PKC beta, PKC theta, Plk1, Pyk2, Ret, ROCK1, ROCK2, RON, Src, Stk6, Syk, TEC, Tie2, TrkA, TrkB, Tyk2, VEGFR1/Flt-1, VEGFR2/Kdr, VEGFR3/Flt-4, Yes, and Zap70.
Among tyrosine kinase receptors, Flt-3 is of special interest. Indeed, 60 to 80% of acute myeloid leukemias (AML) blasts express receptor Flt-3, the receptor for Flt-3 ligand and in a high percentage of acute lymphoblastic leukemias (ALL). Both ligand and receptor have been directly or in collaboration identified at U119 (Hannum et al., Nature 368, pp. 643-648, 1994; Rosnet et al., Genomics 9, pp. 380-385, 1991). Flt-3 mediates differentiation and proliferation of normal hematopoietic stem cells and mediates proliferation and survival signals in AML blasts. Although Flt-3 is most commonly expressed in the wild type form, the leukemic clone of 30 to 35% of patients with AML (Nakao et al., Leukemia 12, pp. 1911-1918, 1996), expresses a mutated form of Flt-3 that contains an Internal Tandem Duplication (Flt-3 ITD) of the juxtamembrane domain coding sequence. This mutation leads to constitutive activation of the receptor and autonomous cytokine-independent growth. It has also been reported that a cohort of AML patients (˜7%) contains mutations in the activation loop of Flt-3 near the amino acid position Asp835 (Flt-3D835) (Yamamoto et al., Blood 97, pp. 2434-2439, 2001). Flt-3 mutations have also been reported at a frequency of 15% in secondary AML and may be associated with disease progression or relapse of AML.
Patients with Flt-3 mutations tend to have a poor prognosis, with decreased remission times and disease free survival. Specific inhibitors of native and/or mutant Flt-3 kinase present an attractive target for the treatment of hematopoietic disorders and hematological malignancies.
Spleen tyrosine kinase (Syk), an intracellular protein tyrosine kinase, is a key mediator of immunoreceptor signalling in a host of inflammatory cells including B cells, mast cells, macrophages, and neutrophils (Wong Br eta/(2004), Expert Opin Investig Drugs, 13, 743-762). Syk is also widely expressed in nonhematopoietic cells like fibroblasts, breast cancer cells, colonic carcinoma cells, hepatocytes, neuronal cells, and vascular endothelial cells (Okamura S et a/(1999), Oncol Res 11, 281-285). Originally, Syk was thought to function primarily in signaling of immunoreceptors such as Fc receptor (FcR) and B cell receptor (BCR). However, recent studies demonstrated the crucial role of Syk in the cell signaling of diverse cellular stimuli including IL-1, tumor necrosis factor-α (TNFα), lipopolysaccharide, and 81-integrin (Yamada T et al (2001), J Immunol, 167, 283-288). For instance, Syk can be activated by TNFα, resulting in MAPK phosphorylation and NF-κB translocation in hematopoietic cell lines (Takada Y and Aggarwal B B (2004), J Immunol, 173, 1066-1077). IL-1-induced chemokine production in fibroblasts of nasal polyps is also mediated by Syk activation (Yamada T et a/(2001), J Immunol, 167, 283-288). Syk has emerged as a potential therapeutic target for treatment of allergic and autoimmune disorders.